The results of a study published in JAMA Psychiatry indicated that infections before age 2 had the greatest effect on a patient’s IQ and risk for adult nonaffective psychosis, and that effect decreased with age.
The population-based, longitudinal cohort study examined 647,515 Swedish men born between 1973 and 1992 (average age 30.73). The population was identified through the National Patient Register, and included men who had been hospitalized with infection from birth to age 13 years. The men were administered an IQ test at age 18 as part of Swedish compulsory military conscription. Men who received diagnoses of psychosis prior to conscription, had emigrated from Sweden, or whose IQ assessment was unavailable were excluded.
Of the 647,515 participants, 23.7% had been hospitalized with infection between birth and 13 years of age. Among those participants, 0.62% (n=4045) were hospitalized in adulthood with nonaffective psychosis and were diagnosed with either schizophrenia (n=1455; 0.22%) or other nonaffective psychosis (n=2590; 0.40%).
Investigators determined infections in the first year of life to be the “sensitive period” for risk of adult nonaffective psychosis. The effect of infection on IQ was also present, and strongest at 1 year, but was statistically insignificant by the age of 5. Additionally, previous research has demonstrated that lower premorbid IQ is also associated with nonaffective psychosis.
Whereas the relationship between early infection and nonaffective psychosis has been well established, the authors also examined the possible influence of genetics through data on sibling and cousin pairs. No relationship was found, suggesting that nonaffective psychosis arises from environmental factors.
A limitation of the study includes the use of hospital admission registers to identify cases of early infection. Only infections that warranted hospitalization were identified, and as all patients were hospitalized, they were also treated. The possible influence of treatment was not examined in this study.
Khandaker GM, Dalman C, Kappelmann N, et al. Association of childhood infection with IQ and adult nonaffective psychosis in Swedish men [published online February 14, 2018]. JAMA Psychiatry. doi:10.1001/jamapsychiatry.2017.4491